This application is related to our concurrently filed application entitled xe2x80x9cProcess for Obtaining Gaseous Nitrogen,xe2x80x9d application Ser. No. 09/810,708 based on German Priority Application No. 10013074.7, filed Mar. 17, 2001.
This invention relates to a process for producing gaseous and liquid nitrogen with a variable proportion of liquid product by low-temperature separation of air in a distillation column system, said system being based on a single column rather than a conventional double column.
Single-column processes are known for the production of nitrogen. In contrast to the double-column process, single-column processes have only a high pressure column (the single column) and no conventional low-pressure column, the latter being normally operated with a reflux of a liquid nitrogen containing stream and a feed of oxygen-enriched air, both the reflux and the feed being obtained from the high pressure column under a lower pressure than the high pressure column. Nevertheless, the distillation column system of this invention may have additional columns beyond the single column, for example for obtaining ultra pure nitrogen or oxygen. Such additional columns are distinguished from the single column insofar as a stream having at least as much oxygen as air is not passed into the ultra pure nitrogen column and a liquid nitrogen stream is not passed into the ultra pure oxygen column.
The xe2x80x9cdistillation column systemxe2x80x9d comprises distillation columns that are connected to one another, but not the heat exchangers or machines such as compressors or expansion engines. In the simplest case, the distillation column system is formed exclusively by the single column.
xe2x80x9cOxygen-enrichedxe2x80x9d is defined here as a mixture of producer gases that has a higher oxygen concentration than air up to virtually pure oxygen. For example, oxygen-enriched fractions have an oxygen content of 25 to 90%, preferably 30 to 80%. (All percentages related here and below are molar percents, unless otherwise indicated.)
The process is used for simultaneously obtaining gaseous and liquid product nitrogen, whereby the proportion of liquid (molar ratio between liquid and gaseous product nitrogen) can be variable. At various times, different stationary operating conditions can thus prevail, in which a varying proportion of nitrogen product in liquid form is obtained. In the extreme case, this proportion can be zero. The operation of the process can then be varied between two boundary cases (1), the maximum gas production (MaxGAN case) with minimum proportion of liquid and (2) the maximum liquid production (maxLIN case) with maximum proportion of liquid and minimum proportion of gas (optionally only liquid production of nitrogen). Furthermore, any value of the liquid portion that lies between the two boundary values for minimum and maximum liquid proportions can also be adjusted.
Known from U.S. Pat. No. 4,400,188 is a process with a nitrogen circuit which comprises cooling compressed feed air in a main heat exchanger and introducing the resultant cooled feed air to said single column operating under pressure; withdrawing a nitrogen-rich fraction from the distillation column system and compressing said nitrogen-rich fraction, at least in a part in a circulation compressor; passing a part of said nitrogen-rich fraction downstream from said circulation compressor to a liquefaction chamber of a condenser-evaporator and condensing said first part of said nitrogen-rich fraction under a pressure higher than the operating pressure of said single column so as to form a nitrogen-rich liquid; passing a liquid, oxygen-enriched fraction from the distillation column system to an evaporation chamber of the condenser-evaporator so as to at least partially evaporate said liquid, oxygen-enriched fraction; passing a first oxygen-enriched gas (234, 533) formed in the evaporation chamber, as ascending vapor into the single column; and withdrawing a second portion of nitrogen-rich fraction at least at times as gaseous nitrogen product, according to the introductory clause of claim 1 is known from U.S. Pat. No. 4,400,188.
A condenser-evaporator, which represents the bottom heating of the single column, is heated with nitrogen, which was brought to a level above column pressure in a circulation compressor. Process cold is produced by an ordinary residual-gas turbine, which is operated with gas from another condenser-evaporator, a top condenser. Such processes with a nitrogen circuit are more advantageous in terms of energy than single-column processes without bottom heating. Because of the circulation, it is believed that a liquid nitrogen product in a variable amount can also be produced in this process, even if this is not described in the publication itself. In such a process, however, difficulties would be encountered if it were desired to vary the proportion of liquid product. If, for example, the liquid proportion is increased, the oxygen concentration and thus the evaporation temperature at the bottom would be decreased with a uniform amount of air. The pressure in the nitrogen circuit must be correspondingly lower, and the circulation compressor thus must be readjusted accordingly. Without changing the circulation pressure, the pressure in the column would increase; in this case, the exhaust pressure of the air compressor must be adjusted accordingly.
The object of the invention is to provide a process of the above-mentioned type and corresponding apparatus, in which in addition to the gaseous nitrogen product, a variable amount of liquid product can be obtained at relatively low cost.
Upon further study, other objects and advantages of the invention will become apparent.
These objects are achieved by withdrawing a portion of the nitrogen-rich liquid from the condenser-evaporator at least at times as a liquid product, operating the evaporation chamber of the condenser-evaporator under a pressure higher than the operating pressure of the single column, and removing a second oxygen-enriched gas from one of the columns of the distillation column system and/or from the evaporation chamber of the condenser-evaporator, machine expanding same and heating the resultant cold gas in the main heat exchanger.
The liquid product can be removed directly in the liquefaction chamber of the condenser-evaporator. It is preferably first depressurized, however, and in this case the flash gas that is produced is separated. The phase separation can be performed, for example, in the single column or in a separate separator.
The operating pressures of the condenser-evaporator and the single column are decoupled by the increased pressure on the evaporation side of the condenser-evaporator. In the case of increasing liquid production, the pressure on the liquefaction side of the condenser-evaporator (nitrogen circuit) does not need to be altered. The pressure on the evaporation side can rather be adjustedxe2x80x94regardless of the operating pressure of the single columnxe2x80x94with uniform evaporation temperature on the lower oxygen concentration without any compression machines having to be readjusted.
The second oxygen-enriched gas, which is provided for active pressure reduction, is preferably produced from the vapor formed in the condenser-evaporator like the first oxygen-enriched gas. The two oxygen-enriched gases have, for example, the same composition. The inlet pressure of the active pressure reduction is notxe2x80x94as is otherwise common in residual-gas turbinesxe2x80x94bonded to the single column- or top condenser pressure, but rather preferably to the evaporation pressure in the condenser-evaporator. The inlet pressure of the turbines within the framework of an increase of the proportion of liquid product can therefore increase analogously to the evaporation pressure. By the correspondingly increased enthalpy difference in the machine expansion of the second oxygen-enriched gas, additional cold is produced, which is necessary for the increased product liquefaction. The increase of the residual-gas amount also increases the production of cold output.
In general, a process for obtaining gaseous and liquid nitrogen is achieved in which the proportion of liquid product can be varied in a very simple way. The proportion of liquid product can be, for example, 0 to 20%, preferably 0 to 16% of the entire nitrogen product, in a total product amount of nitrogen of, for example, 75 to 0%, preferably 75 to 25% of the amount of air. The operating pressure at the bottom of the single column is, for example, 3 to 8 bar, preferably 3 to 5 bar. The pressure difference between the evaporation side of the condenser-evaporator and lower section of the column is, for example, 0 to 5 bar, preferably 0 to 3 bar.
Since the second oxygen-enriched gas ultimately must be derived from the single column, a corresponding pressure-increasing step, which is performed in the invention preferably in the liquid state, for example with a liquid pump, is required. To this end, an oxygen-enriched liquid is removed from the single column and brought to an increased pressure in the liquid state, whereby the second oxygen-enriched gas is produced from the resulting oxygen-enriched liquid that is under increased pressure.
In particular for the case that the distillation column system has only a single column, the oxygen-enriched liquid downstream from the pressure increase forms the oxygen-enriched liquid fraction that is introduced into the evaporation chamber of the condenser-evaporator. The oxygen-enriched liquid is, for example, the bottom liquid of the single column, under which is the evaporation chamber of the condenser-evaporator and it is pumped to at least the increased pressure. The first and the second oxygen-enriched gases, thus the rising vapor for the single column and the fraction that does the work via depressurization, are produced here directly by evaporation of the liquid fraction from the single column.
If it is desired to produce an oxygen product whose purity is higher than that of the bottom fraction of the single column, the procedure is as follows within the scope of the invention. In addition to the single column, the distillation column system has a pure oxygen column. The oxygen-enriched liquid from the single column is passed to the pure oxygen column downstream from the pressure increase. From the lower area of the pure oxygen column, an oxygen-rich fraction is drawn off as a gaseous and/or liquid product and/or intermediate product. The liquid, oxygen-enriched fraction, which is fed to the evaporation chamber of the condenser-evaporator, also is supplied from the lower area of the pure oxygen column. The vapor that is produced in the condenser-evaporator is introduced into the lower area of the pure oxygen column and is used there as ascending vapor. The overhead gas of the pure oxygen column is used in this case in a first part as a working gas of the machine expansion(xe2x80x9csecond oxygen-enriched gasxe2x80x9d) and in a second partxe2x80x94after corresponding pressure reductionxe2x80x94as ascending vapor in the single column (xe2x80x9cfirst oxygen-enriched gasxe2x80x9d). Because of the higher oxygen concentration on the evaporation side of the condenser-evaporator, a higher circulation pressure prevails in this variant than in embodiments in which the evaporation side of the condenser-evaporator is exposed to bottom liquid of the single column.
Stated in simplified terms, an additional mass transfer sectionxe2x80x94named pure oxygen columnxe2x80x94is placed above the condenser-evaporator, and this section is operated under the increased pressure. In this mass transfer exchange section, the liquid that is brought to the increased pressure from the single column is further concentrated in oxygen and more-volatile components are removed from it. Liquid and/or steam from the bottom of the pure oxygen column can be drawn off directly as oxygen product and/or fed to another operating step.
In this embodiment of the invention, the condenser-evaporator is preferably arranged directly at the bottom of the pure oxygen column, but it can also be housed in a separate container. The pure oxygen column is preferably designed as a pure stripping column and contains, for example, 30 to 50, preferably 35 to 45, theoretical plates.
The oxygen-rich fraction can be further purified in the distillation column system by being fed to an additional column for removal of low-volatility contaminants, from whose upper part a pure oxygen product is drawn off. The oxygen-rich fraction is preferably drawn off from the bottom part of the pure oxygen column or from the evaporation chamber of the condenser-evaporator. In the additional column, the ascending vapor is liberated of low-volatility components that are removed accordingly in the pure oxygen product (for example less than 100 ppm, preferably less than 10 ppm of contaminants with a higher boiling point than oxygen; residual contents of up to about 1 ppb can be achieved). Residual liquid from the additional column can be fed back to the pure oxygen column or the condenser-evaporator. The additional column is preferably designed as a pure concentrating (enrichment)column and contains, for example, 10 up to 40 preferably 10 up to 30 theoretical plates.
Reflux liquid for the additional column is preferably produced in a top condenser in which a second oxygen-enriched liquid fraction is at least partially evaporated from the lower part of the single column. The second oxygen-enriched liquid fraction can be drawn off from the single column, for example, together with the oxygen-enriched liquid that is released to the pure oxygen column and brought to an increased pressure.
In all previously mentioned embodiments of the invention, the entire reflux liquid for the single column and optionally the pure oxygen column is preferably produced in the condenser-evaporator. In general, only a single condenser-evaporator is therefore necessary; in the case of an additional column, two condenser-evaporators are necessary.
Air compressors and circulation compressors can be formed by a single machine, namely by a combi-machine, in which several pinion gears are arranged on a shaft, some of which form part of the air compressor and one or more form part of the circulation compressor.
The circulation compressor can be formed at least partially by a compressor that is coupled to the residual-gas turbine, whereby at least a portion of the mechanical energy that is produced in the machine expansion of the second oxygen-enriched gas is used for compression of the first portion and/or the second portion of the nitrogen-rich fraction.
If a nitrogen product of especially high purity is to be produced, it is advantageous if the distillation column system has a pure nitrogen column, whereby a nitrogen fraction from the upper area of the single column in the liquid state is released to the pure nitrogen column, and a pure nitrogen product is drawn off from the lower area of the pure nitrogen column. The pure nitrogen column is used for removing highly volatile contaminants from nitrogen, especially helium, neon and hydrogen. The bottom product of the pure nitrogen column is virtually free of helium, neon and hydrogen (for example less than 10 ppb, preferably less than 5 ppb of highly volatile components that are lighter than nitrogen) and can be drawn off in gas or liquid form. The pure nitrogen column is preferably operated as a pure stripping column and contains, for example, 10 to 20, preferably 10 to 15, theoretical plates.
The nitrogen circuit (first portion of the nitrogen-rich fraction from the distillation column system) can be operated either with very pure gas from the lower part of the pure nitrogen column or with top gas of the single column. It can also be drawn off as a possibly gaseous pressure product (second part of the nitrogen-rich fraction of the distillation system) helium- and neon-free from the pure nitrogen column and/or somewhat less pure from the top of the single column.
The pure nitrogen column preferably has a bottom evaporator, whereby the nitrogen fraction is removed in gaseous form from the single column and is liquefied before it is passed to the pure nitrogen column in the bottom evaporator. By this procedure, no further heating agent for the operation of the pure nitrogen column is necessary. The operating pressure of the pure nitrogen column is somewhat lower (for example by 0.5 to 1.0 bar) than the pressure at the top of the single column. The fraction that is liquefied in the bottom evaporator is depressurized in its operating pressure before being passed to the pure nitrogen column.
In addition, the invention relates to an apparatus for obtaining gaseous nitrogen by low-temperature separation from air with a distillation column system comprising a single column (4), an air compressor, a main-heat exchanger, passage means for feed air to the single column (4) from the air compressor through the main heat exchanger (2);a circulation compressor (9, 1063) for compression of the first portion of a nitrogen-rich fraction (5, 7, 8) from the distillation column system; a circulation line (12, 13), from the outlet of circulation compressor (1063, 9) to a liquefaction chamber of a condenser-evaporator (14); means for feeding a liquid, oxygen-enriched fraction from the distillation column system to the evaporation chamber of condenser-evaporator (14); means for the production of a first oxygen-enriched gas (234, 533) from vapor (232) formed in the evaporation chamber of the condenser-evaporator (14) and for introduction into the single column (4), with a gas production line for drawing off a second portion (19, 20, 1064) of nitrogen-rich fraction (5, 7, 8) as a gaseous nitrogen product, said apparatus further comprising a liquid product line (16, 16), connected to the liquefaction chamber of the condenser-evaporator (14), said condenser-evaporator (14) being inside a container separated from single column (4), and a machine (23) for expanding a second oxygen-enriched gas (221, 521) from one of columns (546) of the distillation system and/or from the evaporation chamber of condenser-evaporator (14).